1. Field of the Invention
The present invention relates to a charged particle beam writing apparatus and a charged particle beam writing method. For example, the present invention relates to a method for calculating a dose of electron beam radiation used for electron beam writing.
2. Description of Related Art
The lithography technique that advances microminiaturization of semiconductor devices is extremely important as being a unique process whereby patterns are formed in the semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. In order to form a desired circuit pattern on semiconductor devices, a master or “original” pattern (also called a mask or a reticle) of high precision is needed. Thus, the electron beam (EB) writing technique, which intrinsically has excellent resolution, is used for producing such a highly precise master pattern.
FIG. 10 is a schematic diagram explaining operations of a conventional variable shaped electron beam (EB) writing apparatus. As shown in the figure, the variable shaped electron beam writing apparatus operates as described below. A first aperture plate 410 has a quadrangular opening 411 for shaping an electron beam 330. A second aperture plate 420 has a variable-shape opening 421 for shaping the electron beam 330 having passed through the opening 411 of the first aperture plate 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector to pass through apart of the variable-shape opening 421 of the second aperture plate 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g. X direction) during the writing. In other words, a quadrangular shape that can pass through both the opening 411 and the variable-shape opening 421 is used for pattern writing in a writing region of the target object 340, on the stage continuously moving in the X direction. This method of forming a given shape by letting beams pass through both the opening 411 of the first aperture plate 410 and the variable-shape opening 421 of the second aperture plate 420 is referred to as a variable shaped beam (VSB) method.
In the electron beam writing described above, the dose of each beam shot is set such that a beam dose at the end of a figure configured by connecting a plurality of shots is to be a threshold value of a dose required for resist pattern resolution. Usually, it is set such that about half the maximum of irradiation energy of a shot dose at the figure end reaches the threshold value. For calculating a dose, one dose equation is used regardless of position of the irradiation. Therefore, when writing a figure configured by connecting a plurality of shots, the dose is set in each shot such that about half the maximum of irradiation energy reaches a threshold value, irrespective of whether it is at the figure end or not.
On the other hand, along with recent tendency of microminiaturization of patterns, the time period of performing writing by the writing apparatus becomes long. Accordingly, it is required to shorten the time period. However, since it needs to enter a calculated dose into the resist in order to write a pattern in accordance with sizes, the conventional method has a limit in shortening the writing time.
Here, in relation to a dose equation used in electron beam writing, there is a method of calculating a dose by changing values, such as a base dose Dbase, used for calculating a dose and a back scattering coefficient η used for correcting a proximity effect, depending on positions in order to correct a phenomenon called a proximity effect etc. However, even in such a method, the dose equation itself used in the method is the same one, and then variables therein are adjusted.
As described above, conventionally, dose of each shot is calculated by using one dose equation which is set in the writing apparatus. When performing irradiation based on an incident dose calculated by the conventional dose equation, all the doses at each of all the regions except for a figure end and for a place on which nothing is written are larger than a threshold value of the resist. In order for all the doses at the figure end to be the threshold value of the resist, all the doses in the vicinity of the figure end need to be larger than the threshold value of the resist.
However, it is enough for all the doses in regions sufficiently distant from the figure end to be about the threshold value. This subject has not been taken into consideration in the conventional method. Therefore, for example, in the case of writing a figure configured by connecting a plurality of shots, if an incident dose of a region inside a figure away from the figure end by a sufficient distance longer than the radius of forward scattering of beam is calculated by using the conventional method, the dose of the region is larger than the threshold value of the resist. That is, when a dose is large, the irradiation time becomes long in accordance with the dose. Thus, as has been described, an excessive dose exists depending on a figure or its irradiation position, and accordingly, there is a problem of taking a writing time longer than needed because of such excessive dose.